This invention relates surgical implants and more particularly to cranio-maxillofacial reconstruction and augmentation. The invention is directed to an improved implant material that is particularly suited for reconstruction and augmentation of cranio-maxillofacial structures and tissues.
A surgeon is sometimes presented circumstances where portions of an patient's face or other areas of the head have been damaged, lost or are missing due to trauma, surgical removal of cancerous or otherwise diseased tissue, or congenital defects. In these instances it is useful to implant a material completely within the body to replace or augment the damaged or lost tissue. In other instances, it is desirable to implant a material to enhance facial features for cosmetic reasons.
A commonly used material for replacement or augmentation of facial and head tissues is a graft taken from other parts of the patient's head, face or body. When the graft is from the patent's own body it is referred to as an autograft. An alternative to autografts are allografts which describe materials harvested from human donor tissue that typically have been processed to minimize infection or triggering an auto-immune response. Another alternative is the use of xenografts which describes implants that originate from animal tissue. Yet a further surgical implant material are referred to as alloplasts which describe implants made from synthetic materials.
Autografts require the material be surgically harvested from another part of the patient's body, and are accordingly subject to the problems of lack of availability and donor site morbidity associated with a second or multiple surgical sites required to harvest the material. Further, in many situations, autografts are subject to shrinkage, resorption or changes in shape that may compromise the desired reconstructive or aesthetic result.
Allografts and xenografts carry the possibility of viral infection transmission or prion transmission, limited availability, and they are also subject to shrinkage, resorption or changes in shape that may compromise the desired reconstructive or aesthetic result.
Synthetic implant materials do not have the problems described above that are associated with allografts and xenografts but have other difficulties. The most common synthetic alloplastic materials are titanium, solid silicone, polymethylmethacrylate (PMMA) otherwise known as acrylic, expanded polytetrafluoroethylene (“ePTFE”), porous polyethylene (“pPE”), and bioactive glass.
Bioactive glass or bioglass is supplied as granules, typically with a particle size range of 90 to 900 microns. Bioactive glass particles have been used as a bone replacement materials and studies have demonstrated that the glasses will aid osteogenisis in a physiologic system. Further, the bonds between the bone and glass, as described in U.S. Pat. No. 4,851,046 has been found to be strong, stable and without toxic effects. The particles may be mixed with saline or body fluids to form a somewhat cohesive granular mixture that can be placed into tissue defect sites. The spaces between the granules allows for fibrovascular ingrowth. In view of the nature of the matrix, bioactive glass does not have the structural integrity of the other alloplasts described here.
Bioglass is commercially available from U.S. Biomaterials of Alachua, Fla. which sells the material having a composition of approximately 45% silicon dioxide, 45% sodium oxide, and the remaining 10% calcium and phosphorus oxide. Bioglass is sold in granular form under the trademark NOVABONE. An exemplary composition and application of bioglass is disclosed in U.S. Pat. No. 6,338,751 to Litkowski et al.
Bioglass has properties that appear to accelerate the rate of fibrovascular ingrowth or bone ingrowth into its macroporous structure. When bioglass is wet with saline or body fluids and placed within the body, it releases by dissolution silicon, sodium, calcium and phosphorous ions into the surrounding area. Over a matter of hours, the calcium and phosphorous ions may recrystallize on the surface of the larger particles in the form of hydroxycarbonate apatite, the physical crystalline structure in bone. As the crystalline layer forms, the body's proteins, including collagen, are attracted to and bind to the crystalline layer. This is thought to be the mechanism that accelerates the growth of fibrovascular tissue or bone within the bioglass macroporous structure.
Bioglass has been combined with solid implant materials such as the implant disclosed in the patent to Bonfield et al, U.S. Pat. No. 5,728,753 which teaches a combination of a polyolefin binder with bioactive glass that results in an implant structure that is strong and maintains flexibility. The bioactive glass is reported to promote interfacial bonding of the implant and surrounding tissues. The patent to Marotta, et al. U.S. Pat. No. 6,299,930 and the patent to Boyan, U.S. Pat. No. 5,977,204 teach the use of bioglass as a coating for implants.
Titanium, silicone, PMMA, ePTFE, and porous polyethylene can be made in rigid or semi-rigid form in a variety of shapes and sizes suited to a variety of reconstructive or aesthetic needs. Examples of such implants include augmentation shapes for recessive chins or cheekbones, stiff sheets to replace missing bone in the orbit or cranium, or even complex customized shapes to replace missing bone in the cranium, orbit maxilla, or other areas. The structural integrity of these materials is an important feature for many implant applications.
Titanium, silicone, PMMA and ePTFE are either solid or in the case of ePTFE, microporous. Microporous in this sense means having pores with an approximate average size under 60 microns in diameter. When these microporous materials are implanted in the body, the body forms a fibrovascular capsule around the implant, effectively walling it off from the body. If the material is soft or pliable, the capsule can contract, changing the shape of the implant. If the space inside the fibrovascular capsule becomes infected, the body's defense system cannot reach the infection, and the implant must be removed. Solid implants are also subject to long term migration, which may alter the desired effect of the implant. Some solid implants have been shown to cause resorption of the underlying bone, again changing the desired effect of the implant. Although solid implants are frequently coated with bioglass to improve interfacial bonding between surrounding tissues and the implant, solid implants do not allow tissues ingrowth and they are not fully integrated with the tissue of the body.
Hydroxyapatite is a natural material used as an implant and is resistant to infection. Hydroxyapatite has a porous structure and allows for tissue ingrowth. However, under some experimental conditions it has been established that hydroxyapitiate interfered with a normal host tissue response and led to chronic mild inflammation that did not completely resolve. Some additional drawbacks to the hydroxyapatite material are that it is abrasive, relatively heavy and must be carved from its natural state to conform to the shape and size of the void or desired shape. Furthermore, hydroxyapatite is relatively brittle and fragile and, due to these inherent mechanical properties, it is difficult to mechanically attach the implant material to the patient's surrounding tissue. Hydroxyapatite may be brittle and can crack at the interface between a screw and the implant material.
Porous polyethylene is a synthetic implant material that can be made with an interconnecting macroporous pore structure. Macroporous in this sense means pores above 100 microns in diameter. A macroporous interconnecting pore structure of porous polyethylene will allow the body to grow new vascularized tissue into the pore structure of the implant, thereby integrating it with the body rather than the body walling it off with a fibrous capsule. Such fibrovascular ingrowth allows the body's immune defenses to operate throughout the implant, to the extent that the implant becomes vascularized. Clinical observations and animal studies suggest that porous polyethylene is less likely to migrate within the body, and is less likely to cause resorption of the underlying bone. These advantages are generally thought to be due to the vascularization of the implant within the open porous structure.
Porous plastic or synthetic resin implants of a surgical grade polyethylene were developed which had a number of advantages over hydroxyapatite. These implants have superior strength, are light weight, and have proven to be effective in many of the applications which had been previously performed by hydroxyapatite materials. Porex Surgical of Newnan, Ga. manufactures such implant materials under the trademark MEDPOR® and markets products designed for implantation in a variety of shapes for a number of applications.
Porous polyethylene is an inert material which has the same advantages afforded by the porous surfaces provided by naturally occurring hydroxyapatite. The plastic is inert, stable and easily can be sterilized. Because the implant is synthetic, an uninterrupted supply of the material is readily available. Further, the material can be easily molded and shaped to appropriately fit a void or be altered to the desired shape. Lastly, because the porous material is flexible and pliable and may be compressed, it allows surgeons to employ coupling methods between the implant and the surrounding tissue. In view of these characteristics polyethylene has been successfully used for a number of years for surgical implant applications. MEDPOR Biomaterial allows for tissue ingrowth because of its interconnecting open pore structure. While the porous nature of the implant allows or permits such ingrowth, the nature of the material does not promote such growth. The firm nature of the material allows carving with a sharp instrument without collapsing the pore structure.
The porosity of MEDPOR Biomaterial is maintained large, with average pore sizes greater than 100 micro-meters and pore volume or the open space within said matrix is approximately in 40% to 60%. The MEDPOR biomaterial is intended for augmentation and restoration procedures in craniofacial applications and is provided in a anatomical shapes, sheets, blocks and spheres including preformed shapes for chin, nasal, malar and mandible augmentation. Blocks for cranial implants may be used for temporal and frontal contouring, as well as for reconstruction of surgical and traumatic defects. The material is also provided in sheets, wedges, and rims for orbital floor, enophthalmos and rim repair. MEDPOR is also made in spherical and conical shapes for enucleation and eviseration procedures.
While porous resin synthetic implants have many of the advantages described above, there are many patients and situations for which the use of synthetic implants may be problematic and can lead to early or late complications. These include: 1) replacement of the eye, where the implant is covered with relatively thin tissues and may be subject to early (within a few weeks) or late (a few weeks to a few years) tissue breakdown over the implant; 2) In diseased or irradiated tissues where healing is less than optimal; 3) where very large implants ace needed; and 4) where there is minimal or inadequate tissue to cover the implant. Particularly in these situations, there is an remains a need to have an implant material with good structural properties, the ability to be manufactured or modified to obtain a variety of shapes, and that has improved fibrovascular or bony integration properties in the body.
Vascularization of the porous implants minimizes the problems of migration and extrusion. Because non-porous implants have a higher incidence of failure due to infection and complications, porous implants are favored. Porous polyethylene implants have the advantage of allowing such tissue ingrowth. While such implants permit such vascularization and ingrowth, it is generally desirable to enhance or improve the vascularization and tissue ingrowth of such implants. While histologic analyses of biopsies from human implants have also demonstrated tissue ingrowth in MEDPOR implants, the clinical significance of tissue ingrowth may vary with the application and implant size. In this regard, magnetic resonance imaging of relatively large implants made of MEPPOR with relatively small surface areas indicates that these implants may not become completely vascularized throughout the implant even one full year after implantation. A method for accelerating fibrovascular ingrowth into such implants would be considered an improvement over the current art.
Accordingly, it is an object of the present invention to provide an improved implant material for cranio-maxillofacial reconstruction and augmentation.
In general, a macroporous implant material with good structural properties and improved fibrovascular integration properties would be considered an improvement over existing implant materials. More specifically, it is an object of the present invention to provide an implant material that has improved bone or fibrovascular ingrowth properties over those of presently available macroporous polyethylene.
It is yet a further object of the present invention to provide an implant material that has improved fibrovascular ingrowth properties and improved structural integrity over that of presently available bioglass implant materials.
It is a further object of the invention to provide an implant material that has improved fibrovascular ingrowth properties and the ability to be molded into a variety of shapes appropriate for cranio-maxillofacial reconstruction and augmentation.
It is a further object of the invention to provide an implant material that has improved fibrovascular ingrowth properties and can be easily modified with a blade or burr to adapt the shape to a particular defect site or to provide an appropriate amount of tissue augmentation.
It is a further object of the invention to provide an implant material that has improved fibrovascular ingrowth properties and that can be fixated to bone or other tissue using presently available fixation techniques.
It is a further object of the invention to provide an implant material that has improved fibrovascular ingrowth properties and that can be molded into customized shapes designed to fit individual patients.
These and other objects and advantages of the present invention will be more readily apparent with reference to the following detailed description and the accompanying drawings.